Many industrial processes and apparatus, as well as household devices, relate to the separation of a liquid phase from another phase. In some instances, particularly when water is the phase present in minor amounts, chemical means may be used to remove the water from the other components. Such means for removing moisture, however, require the replacement and/or regeneration of the reagents used in the process. The reagents employed and the products formed frequently introduce complications relating to handling and disposal. Because of the concomitant cost and, in some instances, inconvenience associated with such processes, physical methods and apparatus have been preferred to chemical means for removal of small amounts of a liquid phase from other phases.
A method of coalescing an immiscible liquid suspended in another phase and a coalescing device, frequently termed a "coalescer", have found widespread use in removing liquid from both the gaseous phase, such as in aerosols, and from suspensions of one liquid in another liquid. Such devices are particularly effective where the volume of liquid removed is small in comparison to the volume of the phase from which it is removed. Typically, the equipment necessary to remove a liquid aerosol from a gas tends to be less complicated than that used to separate two liquid phases in which a first liquid phase is immiscible and suspended in a second liquid phase. This is generally true because in air/liquid suspensions, gravitational effects tend to be more significant while surface energy, surface tension or interfacial tension effects tend to be less significant than with liquid/liquid suspensions.
The spectrum of applications where coalescers have been used to remove minor amounts of a first liquid phase, known as a "discontinuous phase" or "suspended phase", from a second liquid phase in which it is suspended, known as the "continuous phase" or "suspending phase", covers a considerable range of situations. For example, coalescers have been used most often to remove or separate small amounts of moisture from petroleum based fuels, including gasoline, diesel and aviation fuels, such as kerosene; remove moisture from cleaning fluids; separate oil from coolants and parts cleaners; remove oil contamination found in natural bodies of water; separate immiscible solvent systems used in extraction processes, etc.
Numerous mechanisms and models have been proposed to describe coalescence of a droplet of the discontinuous phase from the continuous phase and the ease or difficulty of separation of the immiscible phases. The factors which affect the coalescence process include the physical properties of the phases, such as density, viscosity, surface tension, and interfacial tension (IFT). In addition, the properties of the system, such as drop size, curvature of the interface, temperature, concentration gradients and vibrations also affect coalescence significantly. While any or all of these factors may be significant in a particular situation, properties such as density, drop size and interfacial tension appear to be among the factors which are of most significance and often over which the least control can be exercised in difficult separations of two immiscible liquids. Thus, all other things being equal, where the densities of two liquids differ only slightly, separation becomes more difficult. This is also true of the interfacial tensions of the liquids involved. In those situations in which the droplets are greater than 10.mu. in diameter (primary emulsions) coalescence and separation is much easier to effect frequently with the discontinuous phase settling by gravity after coalescence to form a heterogeneous layer. When the droplets are smaller than 10.mu., particularly less than 1.mu. in diameter, secondary emulsions or secondary hazes result from which the discontinuous phase is much more difficult to coalesce. The latter frequently occurs where the emulsion has been formed by rigorous agitation or the inclusion of a surface active agent. Where emulsification to form the secondary haze occurs purely by mechanical means, coalescence may be accomplished much more readily by conventional coalescence methods and apparatus. Where the secondary haze results from surface active materials, which influence the interfacial tensions of the liquids, separation becomes more difficult.
The type of coalescer employed depends on the difficulty of separation or coalescence, as influenced by the factors identified above. Thus, in some situations, equipment may be very simple, such as those employing baffles, and range to more complex devices containing different types of packing. The type of fluids being separated frequently determines the packing used. Thus, both the shape of the packing material and its composition influence the efficiency of coalescence and separation. For example, the coalescing apparatus used to separate oil and water typically contain tubes, plates, disks, spears, rods, fibers or other internal structures designed to capture oil. Conventionally, glass has been the most often used packing material and while in some instances membranes have been employed in coalescers, as well as the packings listed above, fibers have been the preferred form of packing. Currently, glass fibers seem to have found the most widespread application in coalescers.
In recent years, both household and industrial requirements have led to the demand for purer liquids, including drinking water, solvents, liquids used in industrial processes, and fuels. To satisfy the more stringent specifications required for such materials, requirements have increased with regard to the effectiveness, efficiency and capacity of equipment used to purify these liquids. Manufacturers of such equipment have also striven to provide greater durability and longer interval periods between maintenance, regeneration or replacement of components. In the field of liquid/liquid separation, coalescers have frequently been expected to perform a filtration function to remove particulate matter, in addition to their primary function of coalescing a discontinuous phase.
A typical, conventional coalescing-separating apparatus is illustrated in FIG. 1. The coalescer-separator unit 10 includes a housing 12 having a divided base. An inlet 14 is provided to introduce contaminated liquid through the housing, the liquid then passing through an inlet chamber 16 and thereafter through a coalescer inlet 18 into a coalescer cartridge 20. After passing in an inside-out flow direction through an appropriate packing which defines the walls 22 of the coalescer cartridge, the fluid passes into the body of the housing and thereafter through the walls 32 of the separator cartridge 30 in an outside-in flow path. The external surface of the walls of the separator are provided with a material having a surface energy such that because of the surface tensions of the continuous and discontinuous phases, the liquid forming the continuous phase can pass through the walls of the separator and into the separator body while the liquid which is immiscible therewith is prevented from entering the separator body. In effect, the liquid forming the discontinuous phase, which is coalesced into larger droplets by the coalescer, is repelled in the vicinity of the separator wall 32. The continuous phase which enters the separator cartridge 30, through the separator wall 32, thereafter passes through the separator outlet 28 into the outlet chamber 26 and finally out the housing outlet 24. The coalesced drops of liquid originally in the discontinuous phase flow to the floor or base 36 of the housing unit, situated above the inlet chamber 16 and outlet chamber 26, and out the discontinuous phase outlet or drain 34.
In some industries, the demands for increased capacity have resulted in an increased size of the coalescer units. FIG. 2 represents a plan view of the interior of a conventional coalescing-separating apparatus intended to provide large scale capacity for separation of a discontinuous phase. As may be noted, while the apparatus includes only two separator elements, numerous coalescer units are provided. In this arrangement, fluid enters the inlet 14 of the housing 12 where it then flows, by separate paths, into the inlets (not shown) of the different coalescer units and afterwards through the packing of each coalescer unit 20 into the housing. The liquid then passes into the section of the housing containing the separator elements 30 where the fluid, largely depleted of the discontinuous phase liquid, passes through the walls 32 of the separator units, into the body of the separator units, thereafter passing through the outlet of each of the separator units and out of the housing outlet 24. While the capacity of the apparatus shown in FIG. 2 has been increased as compared to the type illustrated in FIG. 1, such an arrangement results in an uneven flow distribution. That is, a fluid flow or velocity gradient exists between the different regions within the housing. In the arrangement shown in FIG. 2, the gradient exists as a side-to-side gradient in which the row of coalescer units closest to the separators process more fluid than do the remaining coalescer units. At the same time, the separator units have an uneven flow distribution about their circumferences because of their proximity to the coalescer units.
As indicated above, secondary emulsions or hazes present one of the most difficult separation problems where physical methods are used exclusively to separate and remove the discontinuous or dispersed phase. While coalescer-separator devices have been used with varying degrees of success to purify the continuous phase in such applications, the method and apparatus are accompanied by various shortcomings. First, 100% coalescence and removal of the discontinuous phase proves difficult simply because of the very small droplet size of the dispersed phase, which itself may be caused in part by the presence of a surface active substance. Secondly, in those situations in which a surface active material is present, which is a common situation, the change of surface tension attributable to the surface active substances make coalescence difficult, short of removing those surface active substances prior to a coalescing treatment. Third, after a period of use, the surface active substances found in many of these chemically induced emulsions are believed to coat the active surfaces of the coalescer packing, which currently is most often glass fibers, thus "disarming" or rendering the coalescer ineffective. For such reasons, coalescer-separator devices do not provide the degree of purity sought from liquids containing such surface active substances and/or require frequent changing of coalescer elements.
This type of problem is being encountered much more frequently in fuel related industries. Petroleum based fuels tend to pick up moisture, particularly upon storage. Filter-coalescer-separator devices have conventionally been used to remove entrained water from such fuels. In recent years, however, additives, particularly surfactants, have been used in increasing amounts in such fuels. Accordingly, to achieve the same minimal concentrations of moisture, treatments to remove moisture after blending, transporting and storage of such fuels have required more frequent changing of coalescing units. Although the inclusion of phenolic or acrylic resins which primarily act as binding agents for glass fiber packings has had a collateral effect in reducing disarming somewhat but disarming still occurs in high surfactant-containing liquids.